Multilayer structures in which a gas barrier layer containing aluminum or aluminum oxide as a component is formed on a plastic film have been conventionally well-known. Such multilayer structures are used as packaging materials for protecting articles (such as foods) which are susceptible to quality change induced by oxygen. In many cases, such a gas barrier layer is formed on a plastic film by a dry process such as physical vapor deposition (PVD) or chemical vapor deposition (CVD). Such a multilayer structure is used also as a component of a protective sheet for an electronic device to protect the characteristics of the electronic device when the electronic device is required to have gas barrier properties and water vapor barrier properties.
For example, aluminum-deposited films have light shielding properties as well as gas barrier properties and are typically used as packaging materials for dry foods.
Aluminum oxide-deposited films, which have transparency, are characterized by allowing visual recognition of contained substances and by enabling check for foreign matters with a metal detector and heating with a microwave oven. These films are thus used as packaging materials in a wide variety of applications such as retort food packaging.
For example, Patent Literature 1 discloses a multilayer structure having a gas barrier layer containing aluminum, the multilayer structure having a transparent gas barrier layer composed of a reaction product of aluminum oxide particles and a phosphorus compound. Patent Literature 1 discloses a method for forming the gas barrier layer, in which a coating liquid containing aluminum oxide particles and a phosphorus compound is applied onto a plastic film, then dried and heat-treated.
However, in some cases where the present inventors used the multilayer structure of Patent Literature 1 as a packaging material, the appearance of the packaged substance was poor due to insufficient transparency of the multilayer structure. In addition, in some cases where the multilayer structure was used as a packaging material for a retort food, the barrier performance deteriorated as a result of retorting. Furthermore, in some cases where the present inventors used the multilayer structure of Patent Literature 1 in an electronic device, the light transmittance was low due to insufficient transparency of the multilayer structure.
Under such circumstances, a multilayer structure having high transparency and capable of maintaining barrier performance even after retorting has been demanded. In addition, an electronic device including a multilayer structure having high transparency and high barrier performance has also been demanded.